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Solar Powered Aircraft

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    ABSTRACT: HELIOS project was a part of NASA’s ERAST programme (Environmental Research Aircraft and Sensor Technology). It was developed by NASA and Californian company AeroVironment Inc. ERAST programme develops pilotless solar powered airplane technology. The HELIOS construction is based upon its predecessors like NASA’s solar plane Pathfinder, which was successfully tested a few years prior to HELIOS.

    Due to its speed HELIOS was able to fly over the same spot for days or weeks, therefore, in the future such aircrafts could substitute communication satellites. In the test flight on 13th August 2001, HELIOS reached the height of almost 30,000 meters. HELIOS was a slow airplane – if you ride a bicycle fast, you are quicker than HELIOS. It was a remotely controlled plane with no crew weighting less than most cars. However, it flew higher than any other plane powered solely by solar energy.

    In the future, such planes will also be used for interesting research purposes and missions, such as: Fly through the volcano for the purposes of volcano plume study Fly over the North and South Poles Fly for weeks and months at a time collecting scientific data, which varies according to the sun position or the season of the year Explore conditions on Mars. History in brief:| Gossamer Albatros Gossamer Penguin:| | Gossamer Albatross is best known for completing the first completely human powered flight across the English Channel in 1979. The Albatross II was the backup plane for this flight.

    It was equipped with a DC battery-powered electric motor and flight instruments for the NASA research program. NASA completed its flight testing of the Gossamer Albatross II in April, 1980. More about the “Father of Human Powered Flight” Dr. Paul Mac Cready. |   | Pathfinder:| | Pathfinder was a solar-powered, remotely piloted flying wing aircraft used to demonstrate the use of solar power for long-duration, high-altitude flight. Solar arrays provided up to 8 kW of power at high noon on a clear summer day. Pathfinder flew at typical airspeeds of 25 to 35 km/h.

    Pathfinder had a 30 m wing span, weighed 270 kg and was powered by six DC electric motors. It was built primarily of composites, plastic, and foam. |   | Pathfinder Plus:| | In 1998, Pathfinder was modified into the Pathfinder Plus aircraft. Major activities of Pathfinder Plus’s Hawaiian flights included detection of forest nutrient status, forest regrowth after damage caused by Hurricane Inki in 1992, sediment/algal concentrations in coastal waters, and assessment of coral reef health. Pathfinder Plus has a 36 m wing span and weighs about 318 kg.

    Pathfinder Plus has eight DC electric motors, its solar arrays produced about 12,5 kW of power. |   | Centurion:| | Centurion has a 62 m wing span and it is powered by 14 brushless DC electric motors. The Centurion first flew in 1998. As with the Pathfinder/Pathfinder-Plus vehicles, the Centurion was further modified with the addition of a wing extension and a fifth landing-gear pod. Solar arrays have provided up to 31 kilowatts of power at high noon on a summer day to power the aircraft’s motors, avionics, communications and other electronic systems.

    Centurion also has a backup lithium battery system. | HELIOS – technical data:| Wing span: 75, 3 m Length: 3, 6 m Wing thickness: 0. 3 m Height: 2 m, without upper blades of the propellers Wing area: 186. 6 m2 Mass: 600 kg – unloaded plane Allowed mass: up to 930 kg, depends on flight purpose and available energy. Cargo: up to 330 kg depends on measurement equipment weight. Propulsion: 14 DC brushless electric motors (the power of each motor is 1, 5 kW) with two blades, specially designed for high altitude flights.

    The weight of each motor is less than 5 kg. Length of both propeller blades is 1, 7 m. Energy source: Bifacial solar cells – dimensions 1. 25″ x 2. 75″ (Front side efficiency 22 %, backside efficiency 11 %) placed on transparent wings. Energy source in the dark are lithium batteries. Fuel cells will be used as main energy source in the dark in the future. Speed: Typical flight speed is 30 to 40 km/h. The highest speed is 270 km/h. Flight height: Maximal flight height is 30000 m – typical height depends on flight mission and it is typical 15000 to 22000 m.

    Flight: The anticipated autonomy in the future together with fuel cells will reach for uninterrupted flights (several months of autonomy). Materials: All main parts of the plane are made of carbon fibers and Styrofoam. Wings are covered with special designed and produced plastic sheet. Flying wings – From Pathfinder to Helios DESCRIPTION: Silicon, because of its abundant nature (sand) and widespread use in the manufacture of semiconductor devices such as computer chips, quickly became the material of choice for solar cells.

    Over the last several decades, crystalline silicon solar cell technology has become the “workhorse” technology for the majority of PV applications. However, despite its good performance and well-understood manufacturing processes, traditional crystalline silicon solar cell technology remains costly. Subsequently, traditional flat-plate PV systems, which one can see on many rooftops, parking structures, and other facilities, remain costly as well. The industry’s design focus has been to reduce the PV system cost “culprit”–the silicon solar cell.

    Most approaches strive to reduce the silicon material content, many of which involve reducing the thickness of the cells themselves by using very thin films (like paints) of the material. Amonix, because of its extensive background in high-tech semiconductor design and manufacturing, pursued a highly cost-effective approach which involves reducing the area of cell material required to generate a given amount of electricity. This is the high-concentration concept. Amonix has been successful in reducing the silicon cell area by over 500 times with its High-Concentration Photovoltaic (HCPV) system and Mega Module™ design.

    The Cell:  The concept of concentrating sunlight onto a solar cell had been studied for many years; however, one of its main obstacles was that solar cells became very inefficient when exposed to the extreme conditions associated with concentrated sunlight. In 1989, the Electric Power Research Institute (EPRI), the research arm for several hundred domestic utility companies, approached Amonix to solve the solar cell stabilization problem. Using its extensive semiconductor background, Amonix was successful at stabilizing a highly efficient silicon solar cell.

    Further efforts led to the development of a back-junction point-contact silicon solar cell which is the cornerstone of Amonix’s HCPV system. This cell has many superior design attributes including the following: * High Efficiency: Amonix has produced the world’s most efficient silicon solar cell (27. 6% efficiency), also it was manufactured in a commercial environment (an industry first). This performance has been measured and documented by Cal Lab at the Fraunhofer Institute, Germany. High Power Capability:   The Amonix high-concentration silicon solar cell has the ability to generate more power per unit of area than other silicon solar cells. * Both Electrodes on the Same Side:  The Amonix high-concentration silicon solar cell has both electrodes on the same side which allows automated surface mounted robotic assembly and maximum capture of the sun’s energy. * Proprietary Manufacturing Processes:  The Amonix high-concentration silicon solar cell has been designed to allow for high-volume production specifically adaptable to large-volume microelectronic factories for low-cost manufacturing.

    Concentration: Ordinary, one-sun flat-plate solar modules have their entire sun-receiving surface covered with costly silicon solar cells and are positioned at a fixed tilt to the sun. In contrast, Amonix’s systems offer significant cost savings by using inexpensive flat, plastic Fresnel lenses as an intermediary between the sun and the cell. These magnifying lenses focus and concentrate sunlight approximately 500 times onto a relatively small cell area and operate similarly to the glass magnifying lenses we used to burn things with as children.

    Through concentration, the required silicon cell area needed for a given amount of electricity is reduced by an amount approximating its concentration ratio (500 times). In effect, a low cost plastic concentrator lens is being substituted for relatively expensive silicon. Instead of jet fuel, Helios has about 62,000 solar cells across the wing. The solar cells collect energy from the Sun and convert it to electricity, which runs the 14 small motors, which turn the 14 propellers. The propellers are specially designed to pull the aircraft aloft even in the very thin air that’s 18 miles high.

    On August 13, 2001, remote-control pilots on the ground used a computer to fly the Helios Prototype to an altitude of 96,863 feet. That’s over 18 miles straight up! Before the Helios Prototype, the highest recorded flight of any aircraft was about 85,000 feet. This was done in 1976 by the SR-71 spy plane, the fastest jet in the world. Only rockets and rocket-powered airplanes have gone higher. The air we breathe on Earth’s surface is almost 100 times thicker than the air up where the Helios Prototype flew.

    Earth’s atmosphere at this altitude is about as thin as the atmosphere on Mars. This altitude above Earth is so close to space that the sky is almost black, stars shine in the daytime, and the horizon looks curved. As a follow-on to the Centurion (and earlier Pathfinder and Pathfinder-Plus) aircraft, the solar-powered Helios Prototype is the latest and largest example of a slow-flying ultralight flying wing designed for long-duration, high-altitude Earth science or telecommunications relay missions in the stratosphere. Developed by AeroVironment, Inc. of Monrovia, California, under NASA’s Environmental Research Aircraft and Sensor Technology (ERAST) project, the unique craft is intended to demonstrate two key missions: the ability to reach and sustain horizontal flight at 100,000 feet altitude on a single-day flight in 2001, and to maintain flight above 50,000 feet altitude for at least four days in 2003, with the aid of a regenerative fuel cell-based energy storage system now in development. Both of these missions will be powered by electricity derived from non-polluting solar energy.

    The Helios Prototype is an enlarged version of the Centurion flying wing, which flew a series of test flights at NASA’s Dryden Flight Research Center in late 1998. The craft has a wingspan of 247 feet, 41 feet greater than the Centurion, 2 1/2 times which of its solar-powered Pathfinder flying wing, and longer than the wingspans of either the Boeing 747 jetliner or Lockheed C-5 transport aircraft. The remotely piloted, electrically powered Helios Prototype went aloft on its maiden low-altitude checkout flight Sept. 8, 1999, over Rogers Dry Lake adjacent to NASA’s Dryden Flight Research Center in the Southern California desert.

    The initial flight series was flown on battery power as a risk-reduction measure. In all, six flights were flown in the Helios Protoype’s initial development series. In upgrading the Centurion to the Helios Prototype configuration, AeroVironment added a sixth wing section and a fifth landing gear pod, among other improvements. The additional wingspan increased the area available for installation of solar cells and improved aerodynamic efficiency, allowing the Helios Prototype to fly higher, longer and with a larger payload than the smaller craft.

    In addition, project engineers added a differential Global Positioning Satellite (GPS) system to improve navigation, an extensive turbulence monitoring system payload to record structural loads on the aircraft both in the air and on the ground, and radiator plates to assist in cooling the avionics at high altitudes where there is little air to dissipate heat. During 2000, more than 65,000 solar cells in 1,800 groups were mounted on the upper surface of Helios’ wing. Produced by Sun Power, Inc. these bi-facial silicon cells are about 19 percent efficient in the flight regime in which the helios is designed to operate, converting about 19 percent of the solar energy they receive into electrical current. The entire array is capable of producing a maximum output of about 35 kw at high noon on a summer day. The mission to reach and sustain flight at 100,000 feet in 2001 requires use of all 14 motors and minimal ballast to save weight, with the aircraft weighing in at only a little more than 1,600 lbs.

    The four-day mission above 50,000 feet envisioned for the Helios Prototype in 2003 will see only eight motors powering the craft and the addition of the regenerative energy storage system now in development. The system will increase the Helios Prototype’s flight weight to a little over 2,000 lbs. Fewer motors are needed for the long-endurance mission due to the lesser altitude requirements, and the excess electrical energy generated by the solar arrays during the daytime will be diverted to the hydrogen-oxygen fuel cell energy storage system, which will release the electricity to power the Helios after dark.

    Over 570 kW of the 5th generation Amonix system have been manufactured and installed over the last six years. The first three 20 kW units started operating in May of 2000. Since that time, additional units have been manufactured and installed for Arizona Public Service (APS), and for the University of Nevada, Las Vegas and Nevada Power Company. During this time, the units have produced over 3. 7 GHz of grid powerFUTURE EXPANSION:In the future, such planes will also be used for interesting research purposes and missions, such as:

    Fly through the volcano for the purposes of volcano plume study, Fly over the North and South Poles, Fly for weeks and months at a time collecting scientific data, which varies according to the sun position or the season of the year, and Explore conditions on Mars. CONCLUSION: More than 60,000 high efficiency (22. 5% at AM 1. 5) solar cells produced by Sun Power Corporation were used as an energy source for HELIOS.

    The peak power of solar cell array was approximately 30 kW. The total costs for HELIOS solar cells reached around US$ 9 million (200 US$/W). HELIOS was equipped with 14 propeller motors, which were purposely designed for use in very thin air on high altitudes. HELIOS’ wings (in fact, there’s only one wing) were longer than wings of a Boeing 747 or a Lockheed C-5 military transport plane. | | REFRENCES: All Pictures and descriptions: NASA Dryden Flight Research Center |

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